In a flyback switching power converter, a power switch transistor cycles on to conduct a current through a primary winding in the transformer. When the power switch transistor turns off, a secondary winding in the transformer conducts to deliver power to a load. The cycling on-and-off of the power switch transistor subjects it to stress. For example, after the secondary current ramps down to zero, a reflected voltage on the primary winding begins to resonantly oscillate. The voltage stress on the power switch transistor is thus exacerbated if it switches on during a peak of the resonant voltage oscillation.
To reduce this voltage stress, various valley mode switching schemes have been implemented. The power switch transistor is cycled on at the voltage local minimums of the reflected voltage oscillation during valley mode switching operation (each voltage local minimum being designated as a “valley” in valley mode switching). Since the reflected voltage on the primary winding is at a local minimum (or in its vicinity) when the power switch transistor is switched on, valley mode switching operation reduces voltage stress on the power switch transistor. But the frequency of the power switch cycling is also a concern in flyback converters due to the associated switching noise.
Switching noise acts as electromagnetic interference to an electronic system such as a smartphone or tablet being powered by the flyback converter. For example, a touchscreen may be driven with a sensing signal having a certain frequency such as 300 KHz. If the flyback switching noise is within the same frequency band used for the touchscreen sensing signal, the touchscreen operation may have interference from the switching noise resulting from the cycling of the power switch transistor. Modern high-sensitivity touchscreens are particularly susceptible to such interference. For example, touchscreens have been developed such that users may interact with the touchscreens even while wearing gloves. This increased sensitivity for touchscreens heightens their sensitivity to flyback switching power converter switching noise. Given this sensitivity, it is conventional for a touchscreen controller to monitor the noise across potential frequency bands for the sensing signal so that the sensing frequency may be chosen accordingly.
In valley mode switching operation at a steady state load condition, the same valley will tend be used in each switching cycle such that the switching noise becomes concentrated at the resulting constant switching frequency and its harmonics. Despite this concentration, such conventional valley mode switching operation will tend to dither between the selection of a first local minimum and a subsequent second local minimum if the triggering event for the selection occurs around the time of the first local minimum. In one switching cycle, the triggering event may occur slightly before the first local minimum such that the controller selects the first local minimum for the switch on time. In another switching cycle, the triggering event may occur slightly after the first local minimum such that the controller must select for the second local minimum. The resulting dithering results in substantial noise power being spread in the spectral bands between the switching power peaks. The noise energy becomes too widespread across the frequency bands between the harmonics such that a touchscreen controller may have trouble locating a relatively noise-free band in which to drive the sensing signal.
Accordingly, there is a need in the art for switching power converters with improved valley mode switching techniques having reduced noise spreading.